1. Field of the Invention
The present invention relates to a demodulating method and a demodulating circuit in a radio receiver. More particularly, the present invention relates to a method and a circuit for demodulating a reception signal in which selective fading occurs on a transmission path.
2. Description of the Related Art
In the code division multiple access (CDMA) system, information bit sequences are subjected to primary modulation on a transmitting side and then are multiplied by different spreading codes for respective channels to be modulated and transmitted using an identical frequency band. In a demodulating circuit on a receiving side, a reception signal is multiplied by a spreading code identical with the spreading code used for a desired one of the plurality of channels on the transmitting side. Thus, the information bit sequence for the desired channel is taken out and is demodulated. Because the transmission frequency band of the CDMA system is wide, the CDMA system is strong in the selective fading on multi-path, so that information for a lot of channels can be transmitted in the identical frequency band. Also, there is secrecy that it is not possible to demodulate the information if the same spreading code as on the transmitting side is not used. Therefore, the CDMA system is suitable for a multiple access system for a mobile communication system.
FIG. 1 is a block diagram of an example of a conventional demodulating circuit of a receiver in the above-mentioned CDMA system. In FIG. 1, a reception signal as a digital signal with a predetermined frame format is inputted to a path searching circuit 11 and an inversely spreading circuit 12. For example, when the CDMA system is applied to a mobile communication system, this inputted reception signal is the signal to have been transmitted in radio from the mobile terminal and to have been received by a base station. The reception signal is a modulated wave obtained by carrying out phase shift keying (PSK) modulation to a carrier. Moreover, the reception signal is a signal in which one symbol is spread over a plurality of chips with the spreading code. Each of slots of this reception signal is a unique word composed of real part (I signal) of data and an imaginary part (Q signal) of a fixed pattern, as shown in FIGS. 2A and 2B. Each of symbols of the I signal and the Q signal is spread over the plurality of chips with the spreading code. The format shown in FIGS. 2A and 2B is defined in 3GPP (3rd Generation Partnership Project).
Supposing that the inversely spreading circuit 12 of FIG. 1 has M (M is an integer equal to or more than 2) correlating units, the path searching circuit 11 of FIG. 1 produces a delay profile from the above-mentioned inputted reception signal, and allocates M paths of the detected and separated paths to the inversely spreading circuits 12. The inversely spreading circuit 12 inversely spreads the reception signal for each path using a delay quantity obtained from the delay profile calculated by the path searching circuit 11. Through the inverse spreading, the data in each path is changed from the chip unit base to the symbol unit base.
The data outputted from the inversely spreading circuit 12 is supplied to the RAKE synthesizing circuit 13. The estimation of the channel and phase compensation are carried out here for every data in each path. After that, a weighting operation is carried out for a maximum S/N (signal-to-noise) ratio in the RAKE synthesizing circuit and then data for the respective paths are summed. The signal taken out from the RAKE synthesizing circuit 13 is supplied to a decoder 14 and is decoded through metric calculation.
By the way, when the above-mentioned CDMA system is applied to the mobile communication system, an error correcting code which has a high coding gain is introduced. In this case, it is known that the coding gain is maximum in the decoder of the demodulating circuit, when the distortion of the reception signal cancels a time change or fluctuation (for example, xe2x80x9cDigital Communication Receiversxe2x80x9d, (pp. 690-697) by H. Meyr).
However, in the conventional demodulating circuit shown in FIG. 1, the weighting operation is carried out in the RAKE synthesizing circuit 13 such that a S/N (signal-to-noise) ratio after synthesis is maximized. In this case, because the weighting operation to a synthetic output signal is not carried out, the S/N ratio of the input signal of the decoder 14 is not enough large. Therefore, the value of the metric calculation has sometimes received a time fluctuation at the time of the metric calculation by the decoder 14.
Therefore, conventionally, a demodulating circuit is proposed in Japanese Laid Open Patent Application (JP-A-Heisei 10-173629), in which the path timing which should be synthesized is stably extracted, a RAKE synthesis is reliably realized and demodulation with a low error rate can be carried out, when RAKE synthesis is carried out. In the conventional demodulating circuit, an autocorrelation value of the spreading code is calculated. Numerical values obtained by synthesizing a calculation result and a measured reception quality measurement result from the RAKE synthesis signal are set as upper and lower thresholds for an error range of the autocorrelation value. Also, the cross-correlation value of the spreading code and the reception signal allocated for a mobile station itself is calculated. The cross-correlation value and the above-mentioned upper and lower thresholds are compared by the comparing means. If the cross-correlation value is between the upper and lower thresholds, the cross-correlation value is regarded as an invalid correlation value and a corresponding weighting coefficient of a weighting section is set to xe2x80x9c0xe2x80x9d. The weighting section weights the signal obtained by inversely spreading the reception signal using the spreading code. If the cross-correlation value is out of the range between the upper and lower thresholds, the cross-correlation value is regarded as an effective correlation value and the above-mentioned weighting coefficient is set to a predetermined value. After that, the output signals of the weighting section are added, synthesized and outputted to the decoder as the RAKE synthesis signal.
In the conventional demodulating circuit, a wrong timing is never extracted based on the autocorrelation of the spreading code. When the effective reception signal exists in the place of the wrong timing extracted based on the autocorrelation, a threshold is set based on the autocorrelation value, because the cross-correlation value of the reception signal is different from the autocorrelation value of the spreading code. As a result, influence of the autocorrelation value is excluded, and the RAKE synthesis can be carried out, the error rate of the decoder can be suppressed low.
However, the above-mentioned conventional demodulating circuit is complex in the circuit structure, because circuits are necessary such as a section for calculating the autocorrelation value of the spreading code, a section for calculating the quality (SIR) of the reception signal from the signal obtained through RAKE synthesis, and a comparing section for comparing the cross-correlation value and the autocorrelation value of the reception signal.
In conjunction with the above description, a spectrum spreading communication receiver is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-237171). In this reference, a plurality of sets of antenna, correlator and Rake synthesizing circuit are provided. The levels of the output signals of the RAKE synthesizing circuits are compared so as to select one of the output signals having the highest level and the selected output signal is outputted to a demodulating circuit.
Also, a CDMA demodulating circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-335899). In this reference, the output of a phase error compensating section (107) is carried out in a pilot symbol period. It is held for a few symbol periods by a timing adjust function section (110). By using the held phase compensation value, an error generating circuit (109) generates an error vector for every information symbol from signal vectors before and after identification determination. The error vector is sequentially supplied to a tap coefficient control section (111) in the symbol period. The tap coefficient control section (111) updates the tap coefficients of an orthogonal filter in the symbol period. Thus, the tap coefficients are converged in a short time.
Therefore, an object of the present invention is to provide a receiver for a demodulating method and circuit, in which a decode gain is maximized and a RAKE synthesis signal can be demodulated while an error rate is suppressed low.
Also, another object of the present invention is to provide a receiver for a demodulating method and circuit, in which it is possible to improve the performance of a receiver which receives a reception signal of the CDMA system with a simple structure.
In order to achieve an aspect of the present invention, a receiver includes inversely spreading circuit, a RAKE synthesizing circuit, a noise measuring circuit, a weighting circuit and a decoder. The inversely spreading circuit inversely spreads a reception signal for every path using a spreading code to produce path data signals for paths. The RAKE synthesizing circuit synthesizes the path data signals to output a RAKE synthesis signal while carrying out a weighting operation for every path such that the RAKE synthesis signal has a maximum S/N ratio. The noise measuring circuit measures a noise level of each of the path data signals, and calculates a total noise amount for the paths from the measured noise levels. The weighting circuit carries out a weighting operation of the RAKE synthesis signal based on the total noise amount to produce a weighted signal such that a time change of distortion in the RAKE synthesis signal is cancelled. The decoder decodes the weighted signal to produce an information sequence.
Here, the noise measuring circuit may calculate a reciprocal of a summation of the measured noise levels, and the weighting circuit may multiply the RAKE synthesis signal by the reciprocal of the summation of the measured noise levels. Instead, the noise measuring circuit may calculate a summation of the measured noise levels, and the weighting circuit may divide the RAKE synthesis signal with a reciprocal of the summation of the measured noise levels.
Also, the noise measuring circuit may include a plurality of noise measuring devices, each of which is provided for a corresponding path and measures the noise level of one of the data path signals for the corresponding path, and a summing circuit summing the measured noise levels to output the total noise amount. In this case, each of the plurality of noise measuring devices may include a channel estimator estimating a distortion of a corresponding one of the data path signals, a divider dividing the corresponding data path signal by the estimated distortion to produce a divided signal, a first square circuit calculating a first square of the divided signal, a second square circuit calculating a second square of the estimated distortion, a subtracter subtracting one from the first square to produce a subtracted signal, a multiplier multiplying the subtracted signal by the second square, and an averaging circuit averaging the multiplying results by the multiplier.
Also, the receiver may be a mobile terminal, and the reception signal may be a signal from a mobile terminal of a CDMA system.
Also, the receiver may use a maximum ratio RAKE synthesis.
In another aspect of the present invention, a method of demodulating a received radio signal is attained by inversely spreading the radio signal for every path using a spreading code to produce path data signals for paths; by carrying out RAKE synthesis of the path data signals to generate a RAKE synthesis signal; by carrying out a weighting operation for every path such that the RAKE synthesis signal has a maximum S/N ratio; by measuring a noise level of each of the path data signals, and calculates a total noise amount for the paths from the measured noise levels; by weighting the RAKE synthesis signal based on the total noise amount to produce a weighted signal; and by decoding the weighted signal to produce an information sequence.
The weighting may be attained to cancel a time change of distortion in the RAKE synthesis signal.
Also, the carrying out RAKE synthesis and the carrying out a weighting operation may be simultaneously carried out.
Also, the measuring may be attained by calculating a reciprocal of a summation of the measured noise levels, and the weighting may be attained by multiplying the RAKE synthesis signal by the reciprocal of the summation of the measured noise levels. Instead, the measuring may be attained by calculating a summation of the measured noise levels, and the weighting may be attained by dividing the RAKE synthesis signal with a reciprocal of the summation of the measured noise levels.
Also, the measuring may be attained by measuring the noise level of each of the data path signals, and by summing the measured noise levels over the paths to output the total noise amount. In this case, the measuring the noise level may be attained by estimating a distortion of a corresponding one of the data path signals, by dividing the corresponding data path signal by the estimated distortion to produce a divided signal, by calculating a first square of the divided signal, by calculating a second square of the estimated distortion, by subtracting one from the first square to produce a subtracted signal, by multiplying the subtracted signal by the second square, and by averaging the multiplying results by the multiplier.
Also, the receiver may be a mobile terminal, and the reception signal may be a signal from a mobile terminal of a CDMA system.
Also, the receiver may use a maximum ratio RAKE synthesis.